The study, commissioned by the People’s Vote campaign for a second referendum, found GDP over the long term was forecast to be about 4% less than it would have been had the UK stayed in the EU.

It comes as the government prepares to publish its own analysis of the impact of the deal this week,
possibly on Wednesday, to help inform MPs
before they vote
on whether to back it in parliament.

NIESR said the cost to the economy of the prime minister’s deal would be the equivalent of losing about £1,000 a year for every person in the UK.

Garry Young, the director of macroeconomic modelling and forecasting at NIESR, said: “Leaving the EU will make it more costly for the UK to trade with a large market on our doorstep and inevitably will have economic costs.”

The NIESR report found May’s deal would not be as damaging for the economy as Britain leaving the EU without an agreement, which would cost the economy about £140bn over the next 10 years.